The present invention relates to a method of driving a liquid crystal device comprising a liquid crystal material disposed between a pair of substrates opposed to each other. More particularly, the present invention relates to a method of driving a liquid crystal device comprising a ferroelectric liquid crystal disposed between a pair of substrates opposed to each other, said substrates spaced at a predetermined distance from each other and each provided with a transparent electrode and an alignment film formed in this order. The present invention further relates to a liquid crystal device driven by said method.
A twisted nematic (TN) liquid crystal device commercially available at present is driven by active-matrix addressing utilizing thin film transistors (TFTs), and it provides gray scale images. However, the poor product yield and the high process cost in the fabrication of the TFTs are still great problems to be overcome in developing large area display devices.
In contrast to the aforementioned TN liquid crystal devices, those utilizing surface stabilized bistable (SSB) ferroelectric liquid crystals (hereinafter sometimes referred to simply as "FLCs") obviate the need for an external active-matrix addressing driver such as TFTs. Hence, they have attracted much attention from the viewpoint of their potential application to a low cost large-area display device.
Active research and development concerning the application of FLCs to display devices have been undertaken these ten years. FLC displays are superior to other liquid crystal displays, mainly because of the following attributes:
(1) High speed. The electro-optical response of an FLC display is so quick that it yields a speed 1,000 times as fast as that of a conventional nematic liquid crystal display; PA0 (2) Wide viewing angle. An FLC display provides a stable image less influenced by the viewing angle; and PA0 (3) Memory effect. The bistability of an FLC device excludes the need of an electronic or other memory for maintaining an image. PA0 (1) when .epsilon..sub.2 is larger than .epsilon..sub.1 (.epsilon..sub.2 &gt;.epsilon..sub.1), E.sub.eff results larger than E.sub.gap (E.sub.eff &gt;E.sub.gap), because E.sub.gap can be expressed by EQU E.sub.gap =V.sub.gap /d.sub.gap =V.sub.gap /(d.sub.1 +d.sub.2); PA0 (2) when .epsilon..sub.2 equal to .epsilon..sub.1 (.epsilon..sub.2 =.epsilon..sub.1), E.sub.eff is also equal to E.sub.gap (E.sub.eff =E.sub.gap); and PA0 (3) when .epsilon..sub.2 is smaller than .epsilon..sub.1 (.epsilon..sub.2 &lt;.epsilon..sub.1), E.sub.eff results smaller than E.sub.gap (E.sub.eff &lt;E.sub.gap).
Considering a conventional display technique using a ferroelectric liquid crystal disclosed in U.S. Pat. No. 4,367,924 by Clark et al., there is proposed a surface stabilized FLC display device comprising liquid crystal molecules disposed in a panel comprising two flat plates treated to enforce molecular alignment parallel to the plates. The plates are spaced at a distance of 2 .mu.m or less to ensure the liquid crystal material to form two stable states of the alignment field. The quick response of the display in the order of microseconds and the memory effect of maintaining the image have been the subject of intensive research and development.
As described in the foregoing, a bistable mode FLC display is characterized in that it has the following attributes: (1) Flicker-free. The problem of flickers in cathode ray tubes (CRTs) can be overcome by the memory effect of the FLC. (2) Excellent driveability using 1,000 or more scanning lines even in a direct X-Y matrix drive. The FLC display can be driven without using any TFTs. (3) Wide range in viewing angle. Because of the uniform molecular alignment and the use of a narrow-gap liquid crystal panel spaced at a gap corresponding to a half or less of that of a conventional nematic liquid crystal panel, an FLC display can be viewed from over a wider range as compared with the problematic narrow viewing angle of nematic liquid crystal displays which are now prevailing in practical application.
Referring to a schematically illustrated structure in FIG. 28, an FLC display is described below. An FLC display comprises a laminate A composed of a transparent substrate la such as a glass substrate having, in this order thereon, a transparent electrode layer 2a fabricated with an ITO (indium tin oxide; a tin-doped electrically conductive oxide comprising indium) and a liquid crystal alignment sheet 3a fabricated with an obliquely vapor-deposited SiO layer; and a laminate B having a structure similar to that of the laminate A but comprising a substrate 1b provided thereon a transparent electrode layer 2b and an obliquely vapor-deposited SiO layer 3b in this order, provided that the laminates A and B are disposed opposed to each other with a spacer 4 incorporated therebetween to maintain a predetermined cell gap, and in such a manner that the liquid crystal alignment sheets, e.g., the obliquely vapor-deposited SiO layers 3a and 3b, may be opposed to each other. A ferroelectric liquid crystal 5 is then injected into the cell gap between the two laminates A and B.
The FLC displays fabricated in this manner are certainly superior considering the aforementioned characteristics. However, there still is a serious problem to be overcome in realizing displays having sufficient gray scale levels. That is, a conventional bistable FLC display is realized by switching between two stable states, and is therefore considered unsuitable for use in multiple gray scale-level displays such as video displays.
More specifically, in a conventional FLC device (e.g., a surface stabilized FLC device) as illustrated in FIG. 29, the direction of the molecular alignment of a molecule M is switched between two stable states, i.e., state 1 and state 2, by reversing the polarity of an externally applied electric field E. By placing the liquid crystal panel between two crossed polarizers, the change in the molecular alignment can be discerned as a change in transmittance. This is illustrated in the graph of FIG. 30, in which a steep rise in transmittance from 0% to 100% is observed to occur at the threshold voltage V.sub.th with increasing applied electric field. This abrupt change occurs generally within a voltage width of 1 V or less. Furthermore, the threshold voltage V.sub.th depends on the minute fluctuation of the cell gap. Thus, in a conventional liquid crystal device, it can be seen that the transmittance vs. applied voltage curve cannot be set stably within a predetermined voltage range, and that it is extremely difficult or even impossible to realize a gray scale display by simply controlling the applied voltage.
Accordingly, there is proposed an area-modified multi-level gray-scale method (referred to simply hereinafter as an "area multi-gray-level method) which comprises setting the gray scale levels by adjusting the pixel area using sub-pixels or by dividing a pixel electrode into a plurality of portions. There is also proposed a time integration multi-gray-level method which comprises repeatedly applying switching or line addressing within a single field by taking advantage of the fast switching nature of the ferroelectric liquid crystal. However, these newly proposed methods are found still insufficient for a successful multiple gray-level display.
More specifically, in the area multi-gray-level method, the number of sub-pixels increases with increasing number of gray scale levels. It can be readily understood that this method is disadvantageous from the viewpoint of cost to performance ratio concerning the process of device fabrication and the drive method. The time integration method, on the other hand, is practically unfeasible when used alone, and is still practically inferior even when it is used in combination with the area multi-gray-level method.
In the light of the aforementioned circumstances, there is proposed a method which comprises implementing an analog multiple gray-scale level display pixel by pixel. This is realized by locally generating a gradient in the intensity of electric field; more specifically, gray-level display according to the method can be realized by changing the distance between the opposed electrodes within a single pixel, or by changing the thickness of the dielectric layer formed between the opposed electrodes. Otherwise, a potential gradient is provided by using different materials for the opposed electrodes.
Still, however, the fabrication of a practically feasible liquid crystal device capable of displaying an analog multiple gray-scale level image accompanies complicated process steps, and, moreover, it requires a strict control of the fabrication conditions. It can be seen therefore that the cost of fabrication thereby is greatly increased.
Another FLC display device for gray scale display is proposed in JP-A-3-276126 (the term "JP-A-" as referred herein signifies "unexamined published Japanese patent application"). The FLC display device comprises an alignment sheet on which, for example, fine-grained alumina composed of grains from 0.2 to 2 .mu.m in size is dispersed. The switching of the ferroelectric liquid crystal is controlled by adjusting the voltage applied to the portion in which the fine grains are present and that applied to the portion comprising no fine grains. A gray scale display is implemented in this manner.
However, the prior art technology above is of no practical use, because the fine grains used therein are too large in particle size, and because the quantity of the dispersed grains is not clearly stated. Thus, in practice, the designed gray scale display cannot be implemented by following the disclosed technology.
More specifically, for instance, it is greatly difficult to finely reverse the liquid crystal molecules within a single pixel by simply dispersing fine grains from 0.3 to 2 .mu.m in size in a cell having a gap of 2 .mu.m. Moreover, the control of a cell gap in an FLC display is extremely difficult because the FLC display itself utilizes the birefringence mode of the liquid crystal. The failure in strict control of the cell gap results in an uneven coloring. Thus, the technological requirement for the cell above is assumably the same as that for a super-twisted nematic (STN) display device in which the fluctuation in cell gap must be controlled within 500 .ANG..